1. Field of the Invention
The present invention relates to a modulator circuit which suppresses generation of spurious frequency spectrum produced while controlling ON and OFF of the modulated transmittal wave.
2. Description of the Related Art
In a time division communication system or a frequency division communication system in mobile communication, a system which transmits the signal only when a voice signal is present has been known. In view of simplifying the control of transmitter in such system or a time division communication system, a system for controlling ON and OFF of the modulated output wave with a switch circuit has been used. In this case, however, momentary transition of ON and OFF of the modulated output signal generates spurious frequency spectrum. A burst control signal of this case is shown in FIG. 2(a), and a spurious pulse frequency generated in the transmittal output signal is shown in FIG. 2(b). Since the communication systems may be interfered by this spurious frequency, it is essential to suppress generation of this spurious frequency spectrum.
As a method of suppressing such spurious frequency spectrum, as shown in FIG. 1, a carrier is modulated in the modulator 62 by an input pulse signal whose frequency band is limited to the base band frequency by a low-pass filter 61, and the modulated signal is interrupted by a switch circuit 63 controlled by the burst control signal conforming to the allocated time so as to generate a transmittal signal burst. It is also possible to interrupt the carrier input to the modulator 62 according to the burst control signal, in stead of using the switch 63. In this circuit structure, the spurious generated on the transition is reduced by slowing the transition of individual input pulses, resulting in reduced spurious wave generation in the vicinity of carrier frequency. However, in any case, spurious frequency spectrum is still generated in the transmittal frequency band momentarily on the ON/OFF transitions of the transmittal signal.
The spectrum in the transmittal frequency band generated by the input signal pulses is attenuated typically conforming to the curve of S(f)=(sinx/x).sup.2 around the carrier frequency f.sub.o as shown by "a" in FIG. 3. The spurious frequency spectrum generated on ON/OFF transitions of modulated transmittal signal includes a wide frequency component as shown by "b". Therefore, it causes a serious interference in the other communication systems.
In order to eliminate such spurious frequency spectrum, a band-pass filter 64, for example, has been employed in the prior art in the successive stage of switch 63 as shown in FIG. 4, where the parts like those in FIG. 1 are designated by the like numerals. However, even when such band-pass filter 64 is employed, the spurious frequency spectrum in the pass-band of band-pass filter 64 cannot be eliminated. Moreover, in a mobile communication system, a comparatively low speed data is generally transmitted, and therefore a narrow band-pass filter 64 is used, and consequently there is a problem in that an insertion loss becomes large even though the spurious frequency spectrum outside the pass-band can be eliminated.
The modulator 62 explained above is usually formed with a multi-phase phase-modulator or quadrature amplitude phase modulator. Therefore, the modulated signal includes a large change in its amplitude, in other words fluctuating envelope. Accordingly, there is a moment that the envelope becomes zero, and there has been proposed a method that the spurious frequency spectrum is suppressed by controlling ON and OFF of the modulated signal on this moment. (For example, the U.S. Pat. No. 4,644,531 proposed by the inventor of the present invention.) In this method, for example, a same sign which is opposite to the sign immediately before the rise of the burst control signal is continuously given to at least two bits immediately after the rise of the burst control signal, and thereafter the signal is inverted. Or, the sign of at least two bits immediately before the fall of the burst control signal is set opposite to the sign of the preceding bits, and the sign is then inverted immediately after the fall of the burst control signal. Thereby, the signal is controlled ON and OFF at the moment at which the amplitude of the modulated signal becomes zero, and the spread of the spurious frequency band generated thereby can be reduced.
Meanwhile, since the modulated signal whose amplitude does not become zero receives less influence of nonlinearity distortion of the amplifier, the amplifier can be simply structured as well as reduced power consumption can be realized. Accordingly, this method provides a merit of realizing reduction in size of a mobile station in the mobile communication system, and is preferably employed. An example circuit is hereunder explained.
FIG. 5 is a block diagram of an offset 4-phase PSK (Phase Shift Keying) modulation circuit of a prior art. In this figure, numeral 71 designates a 4-phase phase-modulation circuit; 72, a phase shifter for delaying an input signal pulse for a 1/2 bit period; 73, a serial/parallel converting circuit; 74 and 75, low-pass filters; 76 and 77, modulator units each formed with, for example, a balanced mixer; 78, a combiner; 79, a phase shifter for shifting the carrier for .pi./2; 80, a carrier oscillator; and 81, a switch.
An input data is converted to parallel data I and Q by serial/parallel converter 73, and the Q channel output data is shifted for 1/2 bit by phase-shifter 72. The switch 81 is controlled by a burst control signal and thereby the carrier applied to the modulator units 76 and 77 is controlled ON and OFF. When the switch 81 becomes conductive, the carrier from the carrier generator 80 is applied to the modulator units 76 and 77; and the modulated signals, as the respective outputs, are summed in the combiner 78 to become a transmittal signal. When the switch 81 opens, the transmittal signal is cut. FIG. 6(a) and FIG. 6(b) respectively show modulating signals input to the modulator units 76 and 77 through the low-pass filters 74 and 75 of the I channel and Q channel. Since these signals are shifted by 1/2 bit period with each other, the transmittal signal combined by the combiner unit 78 is shown in FIG. 6(c). Namely, the modulated transmittal signal has a small change in amplitude and does not include any moment at which the amplitude becomes zero.
FIG. 7 is a block diagram of an FSK (Frequency Shift Keying) circuit of the prior art. Numerals 84 and 85 designate low-pass filters; 86 and 87, modulator units each formed, for example, with a balanced mixer; 88, a combiner; 89, a .pi./2 phase-shifter; 90, a carrier oscillator; 91, a switch; 92, a serial/parallel converting circuit; 93, an quadrature amplitude phase modulation circuit; and 94, a logic processing circuit.
An input data is converted to parallel data I and Q by the serial/parallel converting circuit 92, applied to the logic processing circuit 94, and then converted to phase-component signals cos .phi. and sin .phi.. These phase-component signals cos .phi. and sin .phi. are applied to the modulator unit 93 so as to be quadrature-modulated. Thereby, an FSK-modulated signal is output.
A .pi./4-shift QPSK (Quadrature Phase Shift Keying) signal can be obtained by providing a mapping circuit (not shown in the figure) in place of the logic processing circuit 94, thus, after being logic-processed without changing the phase by 180 degrees the data is input to the modulator unit 93. The amplitude of thus modulated signal does not become zero. Therefore, the ON/OFF control of the modulated output signal by these modulation systems causes the following problems. Namely, as described above, the phase-modulated signal and quadrature amplitude phase modulated signal, each of which largely changes the envelope of the modulated signal on the transition of input signal, can be made zero in the amplitude, on ON/OFF of the modulated signal by logical processing of the input pulse signal, accordingly generation of spurious frequency spectrum can be suppressed. However, as mentioned above, in the modulation circuit shown in FIG. 5 or FIG. 7, since the modulated signal output has no moment at which the envelope becomes zero, it is inevitable that the widely spreading spurious frequency spectrum is generated by the ON/OFF transitions of modulated signal.
In order to eliminate such disadvantage, it has been proposed that the input data is previously grouped as a burst, namely as a group of pulse train, transitions of leading edge and trailing edge are dulled by a filter. However, in such a communication system that requires a sufficient time is between the burst periods, the ON/OFF control can be carried out during the pause period between the bursts. But, since the pause period becomes longer and thereby transmission efficiency is lowered, such a method is often difficult to be applied to the mobile communication system.